THE INTERNAL STRUCTURES OF PLANTS. 201 



CELL-PRODUCTION. >The growth of plants takes 

 place, either by the expansion of existing cells, or 

 by the formation of new ones, and principally by 

 the latter method. "The contents of the cells of 

 the growing part divide into two, and between the 

 halved contents there forms a thin layer, which di- 

 vides each cell into two distinct cells. The new cells, 

 then, increase in size until they become as large as 

 their parent-cell, when they each divide again, and the 

 process is repeated. The process is modified according 

 as the cells are to lengthen or to remain short." 



The rate at which cells are formed may be gath- 

 ered from such a fact as the growth of a huge puff- 

 ball, sometimes nearly a foot across, in a single day. 

 In this sudden growth it has been estimated that 

 300,000,000 or 400,000,000 cells are produced in an 

 hour. Century -plants, growing in conservatories, 

 after many years produce a flowering stem six inches 

 in diameter. The entire vigor of the plant is devoted 

 to the growth of this stalk, which ascends at the rate 

 of a foot in twenty-four hours. Estimating the cells 

 at -gfa of an inch in diameter, there are formed more 

 than 20,000,000,000 a day. 



EXERCISE LXIV. 

 Vessels or Ducts, and Fibres. 



EXPERIMENT. Take some of the boiled pulp of 

 any soft vegetable substance, as rhubarb, that can 

 be picked to pieces with needles. Put a bit of this 



